A crankshaft is manufactured by molding a forged product including fins by die forging through pressing a heated material with upper and lower molds, thereafter removing the fins, and performing shot blast processing. The crankshaft manufactured in these manufacturing processes is subjected to machining so as to be incorporated appropriately in being incorporated into an engine of an automobile, or the like.
FIG. 1 is views schematically illustrating one example of a crankshaft (crankshaft for inline-four engine), and is a front view seen from a direction of a rotation center axis L of a crankshaft S and a side view seen from a direction orthogonal to the rotation center axis L.
The crankshaft S includes a plurality of pins S1 for fixing connecting rods (not illustrated) provided in positions at a predetermined angle around the rotation center axis L, a plurality of counterweights S2 for taking a rotation balance, and a plurality of journals S3. A shape of the pin S1 is a columnar shape with a position separating from the rotation center axis L of the crankshaft S being a center. A shape of the journal S3 is a columnar shape with the rotation center axis L of the crankshaft S being a center. A cross-sectional shape of the counterweight S2 is a symmetrical complex shape.
As described above, since the crankshaft is complex in shape, during forging, variations in material dimensions, unevenness of a material temperature, variations in forging operations, and the like sometimes cause a defect referred to as an underfill in which the material is not filled to an end portion of the mold, a bend or a twist over the entire length of the crankshaft, or a tilting of the counterweights. In addition, contact with conveyance equipment or the like also sometimes causes a dent flaw during handling the crankshaft. In the manufacturing processes of the crankshaft, in order to detect the underfill, the bend, and the twist of the crankshaft subjected to the shot blast processing, and the dent flaw, before performing the machining, an actual shape of the crankshaft is inspected in comparison with a reference shape to judge acceptance or non-acceptance.
As criteria of judgment of acceptance or non-acceptance of the crankshaft,
(a) a bend and a twist of a crankshaft and positions of counterweights being each within a predetermined allowable range,
(b) absence of an underfill with a depth not allowing sufficient machining stock to be secured and a dent flaw,
can be cited.
The reason why a bend and a twist of a crankshaft and positions of counterweights being each within a predetermined allowable range is one of the criteria of judgment of acceptance or non-acceptance as described in the above (a) is that a large displacement of an installation angle of the pins or the positions of the counterweights from predetermined angle or positions due to a large bend or a large twist of the crankshaft makes it difficult to achieve dimensional accuracy or weight balance as a final product even though any processing is performed in a post-process.
Further, the reason why absence of an underfill with a depth not allowing sufficient machining stock to be secured and a dent flaw is one of the criteria of judge of acceptance or non-acceptance as described in the above (b) is that too small machining stock leads to small room to perform machining in the post-process and makes it difficult to achieve the dimensional accuracy or the weight balance as the final product.
A conventional method for inspecting the crankshaft has been the one in which plate gages formed so as to match with reference shapes of the pins and the counterweights are each put to the pins and the counterweights of the crankshaft to be inspected, to measure gaps between the plate gages and the pins and the counterweights with a scale, and if dimensions (shape errors) of the gaps are each in the allowable range, the crankshaft is judged acceptance. Since this method is performed manually by an operator by using the plate gages formed so as to match with the reference shapes of the pins and the counterweights, it has had problems of not only occurrence of an individual difference in inspection accuracy but also a time-consuming inspection. Therefore, in order to perform an accurate inspection automatically, various inspection methods for the crankshaft have been proposed (for example, refer to Patent Literature 1 to Patent Literature 4).